The invention is directed to the production of dihydroxybenzenes as well as their monoethers by nuclear hydroxylation of the corresponding monophenol or phenol ether with hydrogen peroxide.
Important dihydroxybenzenes are derivatives of phenol, the naphthols, and also derivatives of anthracene and phenanthrene. They are employed in the production of dyestuffs, in the production of synthetic resins, in photography, and for the production of important plant protectives.
Their production, therefore, has long been the object of thorough investigations. The hydroxylation has been carried out both with hydrogen peroxide itself as well as with hydroperoxides, peroxides, or even per acids such as, e.g., performic acid or peracetic acid.
Nevertheless, hydrogen peroxide was preferred since it is the most readily available and since with percarboxylic acids, hydroperoxides and peroxides side reactions occur (European published application No. 0027593).
There was always present a catalyst in these hydroxylations. This catalyst can be a metalloid such as sulfur, selenium, tellurium, phosphorus, arsenic, or antimony in elemental form (German OS No. 2348957) or there can be used boron compounds (German Pat. No. 1543830).
Various processes operate with transition elements in the form of their ions (German OS No. 2162552), especially with iron ions (German OS No. 2162589 or German Pat. No. 2407398) or cobalt ions (German AS No. 2341743), or even with the corresponding oxides (Milas U.S. Pat. No. 2,395,638).
Besides, there are employed strong acids such as sulfuric acid, sulfonic acids (German OS No. 2138735, German AS No. 2410742, German AS No. 2410758, German AS No. 2462967), or a mixture of sulfuric acid and phosphoric acid (German OS No. 2138735), there are also mentioned in the last named published application organic acids such as, inter alia, trichloroacetic acid or tartaric acid.
The already mentioned percarboxylic acids likewise serve as catalysts (French Pat. No. 1479354). In all of the mentioned catalysts, it is a matter with the catalysts being solid or liquid materials. Hydrogen peroxide, as preferred oxidation agent, for the most part is employed in aqueous solutions of various concentrations up to very high concentrations which have the danger of explosion; thus, the process according to German Pat. No. 2064497 operates with solutions which only contain 5 weight % water, but even at this highly concentrated hydrogen peroxide the yield of dihydroxy derivatives was only 70% and was reduced considerably according to the dilution of the hydrogen peroxide.
Additionally, in these and also in other processes, the operation must be carried out with a very large excess of the phenol to be hydroxylated in order in general to obtain the above-stated yield. If this excess is reduced, e.g., from 20 moles to 10 moles per mole of hydrogen peroxide, then the yield is reduced drastically despite the higher concentration of hydrogen peroxide.
However, as is known, this type of excess of a reactant, which must be recycled, requires additional industrial expense; above all in regard to the size of the apparatus employed.
Since care is always taken to avoid large excesses of one component as far as possible, there have been attempts to avoid employing aqueous solutions of hydrogen peroxide.
Thus, different solutions of hydrogen peroxide in organic solvents have already been used. For example, according to the process of German Pat. No. 2410758, there are preferably employed hydrogen peroxide solutions in derivatives of phosphoric acid or phosphonic acid, namely in the presence of a strong acid, such as sulfuric acid (100%) or fluorosulfonic acid.
However, these highly concentrated strong acids have the disadvantage that their separation from the reaction mixture creates difficulties (German AS No. 2658943), above all since their concentration in the reaction mixture has a considerable influence on the length of the reaction.
The excess of phenol was indeed reduced somewhat in contrast to this in the process of German AS No. 2064497, but this did not outweigh the disadvantage of the strong acids.
An additional difficulty in the process of German Pat. No. 2410758 in the working up of the reaction mixture was produced by the presence of the water formed after the reaction with hydrogen peroxide.
Since the solvent for hydrogen peroxide employed in part is higher boiling than the phenols employed and these frequently, above all phenol itself, form an azeotrope with water whose boiling point is below that of the organic solvent, it was highly problematic that a trouble-free separation of the phenols from the reaction mixture could be attained.
Therefore, other ways were tried, first to manage without catalyst, i.e., above all without the strong acids. Since the catalysts above all were needed for the activation of hydrogen peroxide, the process of German AS No. 2658943 was operated with organic solutions of peracetic acid. An additional catalyst was not used.
Entirely apart from the fact that the mentioned process presupposes a complete plant for the production of an organic percarboxylic acid, which first is obtained from hydrogen peroxide and carboxylic acid, and thereupon is produced by extraction of this so-called "equilibrium acid" from its aqueous solution, it has been shown a stated good selectivity and good yield was only possible in the presence of additional peracid stabilizers (German OS No. 2364181; European OS No. 0027593). Also, the attempt to produce pyrocatechol and hydroquinone without catalyst with gaseous hydrogen peroxide could be carried out only poorly on an industrial scale because of the danger of explosion (Japan published application No. 24056/1974).
From what has been said above, the result is that processes in which hydrogen peroxide is used as the most readily accessible hydroxylating agent do not make possible any entirely satisfactory process for the industrial production of dihydroxybenzenes.
Therefore, in recent times, there have only been developed processes which do not directly use hydrogen peroxide and in part for this reason require high industrial expense.
The subject matter of the invention, therefore, is carrying out the nuclear hydroxylation of substituted phenols or their ethers with hydrogen peroxide in the presence of catalysts in an industrially simple manner and with very good yields.